kernel_multiple Computes the sensitivity kernel for a wave traveling
  from S to R.
 
  CALL : 
     [K,RAY,Gk,Gray,timeS,timeR,raypath]=kernel_multiple(Vel,x,y,z,S,R,T,alpha,Knorm);
 
  IN : 
     Vel [ny,nx] : Velocity field
     x [1:nx] :
     y [1:ny] :
     z [1:nz] :
     S [1,3] : Location of Source
     R [1,3] : Location of Receiver
     T : Donminant period
     alpha: controls exponential decay away ray path
     Knorm [1] : normaliztion of K [0]:none, K:[1]:vertical
 
  OUT :
     K : Sensitivity kernel
     R : Ray sensitivity kernel (High Frequency approx)
     timeS : travel computed form Source
     timeR : travel computed form Receiver
     raypath [nraydata,ndim] : the center of the raypath 
 
  The sensitivity is the length travelled in each cell.
  
 
  See also : fast_fd_2d
 
  TMH/2006